Variable beam tilt is an important tool for optimizing radio access networks for cellular telephony and data communications. By varying the main beam pointing direction of the base station antenna, both interference environment and cell coverage area can be controlled.
Variable electrical beam tilt is conventionally performed by adding a variable linear phase shift to the excitation of the antenna elements, or groups of elements, by means of some phase-shifting device. For cost reasons, this phase-shifting device should be as simple and contain as few components as possible. It is therefore often realized using some kinds of variable delay lines. In the description, the terms “linear” and “non-linear” should be understood to refer to relative phase over multiple secondary ports of a multiport phase shifting network, and not the time or phase behaviour of a port in itself.
Conventional multi-port phase shifters, with one primary port and a number N (N>1) secondary ports, are implemented with linear progressive variable phase taper over the secondary ports. In addition to the linear progressive phase taper, fixed amplitude and phase tapers are often used as a means for generating a tapered nominal secondary port distribution.
FIGS. 1a and 1b illustrate a conventional phase shifter 10, with one primary port 11, and the phase shifter generates in down-link linear progressive phase shifts over four secondary ports 121-124. A variable-angle “delay board” 13 has multiple trombone lines 14, one for each secondary port 121-124. The trombones lines 14 are arranged at linearly progressive radii. By a proper choice of junction configurations, line lengths, and line impedance values, the nominal phase and amplitude taper of the phase shifter can be controlled, for example to achieve uniform phase over the secondary ports as indicated by “0” in FIG. 1a. By changing the delay line lengths (i.e. the length of the trombone lines 14), in this case by rotation of the delay board 13 relative to a fixed board 15, the secondary ports 121-124 experience linear progressive phase shifts as indicated in FIG. 1b. In up-link, the secondary ports 121-124 receive signals from an antenna (not shown) which are combined within the phase shifter to a common receive signal at the primary port 11.
The use of non-linear phase-shifting devices for controlling electrical down tilt has been contemplated, such as mentioned in U.S. Pat. No. 5,798,675, by Drach, U.S. Pat. No. 5,801,600, by Butland et al.
A system for tilt-dependent beam shaping using conventional linear phase shifters is disclosed in JP 2004 229220. The system has different beam width depending on the tilt angle, but this is achieved by a tilt angle control section (41) in combination with a vertical beam width control section (42) in the base station controller (4), see FIG. 6 in JP 2004 229220.
Traditionally, base station antennas have had a variable beam tilt range of approximately one beamwidth. This together with the fact that most mobile connections today are circuit switched voice with a fixed requirement on bit-rates, has not triggered any interest in improving the Signal-to-interference+noise ratio (SINR) close to the antenna. Normally it is good enough.
For particular cell configurations, e.g. highly placed antennas in combination with small cells, the need for using antennas with large beam tilt is greater. For antennas with conventional narrow elevation beam radiation patterns, the large beam tilt causes users close to the base station to experience a lower path gain than users close to the cell border, since the difference in path loss for the near and far users is smaller than the difference in directive antenna gain. For packet-based data communication this is not optimal usage of the available power. Therefore, for antennas with large beam tilt, some degree of radiation pattern null-fill below the main beam, or even some cosec-like beam-shaping is desirable.
In large cells, on the other hand, when no or small beam tilt is employed, the antenna pattern should be optimized for maximum peak gain. The path gain for the users at the cell border will anyway be smaller than for users closer to the base station because the path loss varies rapidly with vertical observation angle in the case of large cells and observation angles close to the horizon.